Light emitting diodes are small and highly effective in power consumption, and emit vivid color light. Dissimilarly in light bulbs that heat a filament for light emission, there is no concern about bulb burnout and so on in light emitting diodes. In addition, light emitting diodes have features that have very high responsibility, and high resistance against vibration and against repetition of ON/OFF switching. Since light emitting diodes have these advantages, and thus are used as various kinds of light sources.
Light emitting diodes emit light in a particular wavelength range. For this reason, a light source has been developed which converts a part of light that is emitted by a light emitting diode into a different wavelength by a phosphor, and mixes the light that is converted into the different wavelength by the phosphor and the light that is emitted by light emitting diode to emit mixed light. This type of light source can provide light of various colors different from the light emitting diode depending on selection of the colors of phosphor light emission. In particular, a light source that emits white range light is widely used for general lighting, a display, an LCD backlight, and so on. Accordingly, there are needs for a phosphor that can be used for a white group light emitting device in combination with a light emitting diode. In the case of a light source that includes a light emitting diode, white light is provided by mixing colors of light of a blue light emitting diode and a phosphor based on the additive process. In this type of light source, blue light that is emitted by a light emitting chip of a light emitting diode excites a phosphor. The phosphor absorbs the blue light from the light emitting chip, and thus emits yellow light. The yellow light of the phosphor and the blue light of the light emitting chip have a complementary color relation ship. Human eyes recognize the mixture of the yellow light and the blue light as white light. Light emitting diode type white light sources are produced which combine a blue light emitting chip and a phosphor based on this additive process.
An example of the phosphor used in this application is an oxisulfide phosphor composed of Y2O2S:Eu. In addition, a nitride phosphor composed of Ca2Si5N8:Eu has been developed (see Patent Document 1).    Patent Document 1: International Publication Pamphlet WO01/40403    Patent Document 2: Japanese Patent Laid-Open Publication No. 2003-147351